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Friday, February 28, 2014

Jack - WA7KMR and I spent some time in his Lab, using his Test Equipment to characterize the first few modules of my homebrew implementation of Farhan's Minima Transceiver. We tested my; RF Mixer, Crystal Filter and IF Amplifier (see previous post).

Jack at the Controls

We first connected each module as a standalone device to ensure each worked as expected. We were both most interested in the Crystal Filter performance measurements. My previous measurement attempts with the AIM-4170B were inconclusive, other than the shape of the bandpass.

Crystal Filter

With Jack's Lab Equipment we measured the bandpass and insertion loss of the Crystal Filter. The bandpass is 6KHz, and insertion loss is about 3db, with about 2db of ripple. With some tweaking, I think the ripple could be reduced.

We later checked the operation of the standalone RF Mixer, and then the IF Amplifier. The RF Mixer worked similar to one of Jack's known mixer. The amp provided about 30db of gain at 20MHz.

We connected all three modules together and I very pleased with the results.

RF Mixer, Crystal Filter, and IF Amplifier

Output of the IF AmpHorizontal = 20MHz Center, 5KHz / DivVertical = -30dbm at top line, -110db at bottom, 10db / DivLO was 200mV RMSThe RF Signal was Detectable down to -90dbm

The next module to be constructed would be a BFO Mixer, which should be simple as it uses a similar layout as the RF Mixer. Then an AF Amplifier will be needed, which will be constructed in similar fashion. Also, a Low Pass filter will be constructed and connected before the RF Mixer.

This configuration only implements a Receiver, but with a few relays, bidirectional amplifiers, full Transceiver functionality should be possible (similar to Farhan's Minima Transceiver).

When completed I plan to use my Parallax Propeller microprocessor for control, and generate the RF for both VFO and BFO sources. This is where my implementation radically departs from Farhan's Minima Transceiver, but then remember, . . . this is just an experiment !

Note: In the photo above, one of the two RF Mixer coils has be replaced with a much smaller core as originally planned (see previous post). The other (larger) core will be replaced when time is available.

Sunday, February 23, 2014

Somewhere (link found) I read that for Homebrew Crystal Filters the HC-49US (short) crystals do not have as high of "Q" as the standard HC-49U (standard) crystals, but as with all of my projects "small is better". This experiment is being done to see what I can do with the short crystals.

To try to find matching crystals, I used an AIM-4170B Analyzer to sort the crystals into six, 5-digit groups (19.991 - 19.996MHz). The crystals sorted into a typical standard bell distribution. I think the measured frequency, which is lower than the expected 20MHz, is due to calibration, and/or because the crystals are not actually operating in an oscillator circuit.

For this build, I decided to use the 19.993MHz group, as it provided more crystals to further select from.

Initial Sorting of 20MHz Crystals

The Analyzer display provided the Resonate Frequency and other information.

AIM-4170B 20MHz Crystal Plot

I re-tested each crystal of the 19.993 group, sorting and recording the frequency to 7-digits. A green tape label was attached to each crystal to make sorting easier.

Sorted Crystals

I selected a set of eight crystals for my filter, several of which had the same values.

Selected Crystals

The selected crystals have the following frequency values:

2 - 19.99265

1 - 19.99266

2 - 19.99267

1 - 19.99269

2 - 19.99271

A double sided 1 x 2 inch PCB was created to mount the crystals and capacitors, the Toner Transfer Method was used to make the PCB.

20MHz Crystal FilterToner Transfer PCB

This is the results after Cutting, Drilling and Loading the board.

Completed 20MHz Crystal Filter

I think the AIM-4170B is NOT the best instrument to measure overall performance of a Crystal Filter, But, initial testing indicated there were some major "spikes" in the frequency response.

I initially loaded the PCB with two parallel 50pF capacitors at each of the five locations where Farhan's circuit called for a 100pF. The goal was to parallel two capacitors to lower series inductance and/or provide the option of replacing one-of-each with a variable capacitor (if needed).

Due to the observed "spikes", I replaced one-of-each set of two capacitors with a 4-47pF NP0 variable capacitors.

Tuning Caps Installed

With a little tweaking, the "spikes" were removed, and this is resulting SWR plot with a 50 ohm Load on the output. But still, the resulting curve is less flat than I expected, . . .

SWR with 50 Ohm Load

The real test and performance evaluation will need a Spectrum Analyzer and/or a measurement done within a fully constructed receiver.

More Crystal Filter fun to follow :-)

UPDATE: Feb 23, 2014 14:04
I just received an e-mail from Jack, we should be able to evaluate the Filter soon.

Hi Eldon,Nice looking XTAL filter.If your design is around 50 ohms in and out then my equipment should do the job.I expect to have the interconnect cable for my Spectrum Analyzer to Tracking Generator by next Tuesday.Assuming both pieces of gear work as planned I will be testing about 4 RF filters I have on hand and your XTAL filter would be a lot of fun to test.I can also provide a print out of the filter response curve. A camera shot of the CRT also works.Jack

Thursday, February 20, 2014

My Son and I are working on a project to automate; 20 air driven punches, with a similar strategy that was used by the old "pin printers", but our project will be on a much larger scale.

The planned punch printer will be much larger and much more powerful.

Air will be used to drive the punches and will be controlled via 12Volt air solenoids. I have previously posted the a description of the single test FET Driver that will be used, and which will be controlled via a microprocessor. The planned circuit for the finial product will consist of three stacked boards with 8 FET Drivers per board. The Toner Transfer of the Prototype board is shown here.

Eight Circuit FET Driver Board

a Toner Transfer Prototype

To test the FET Driver, I needed a microprocessor to produce a test signal that would be easy to use.

From a previous project, I have an "Adafruit Trinket" microprocessor programmed to produce a "CQ" on one of it pins. A simple jumper was used to connect the Trinket to the FET Driver.

Trinket Sending CQ

The results was "CQ" being taped out by the air valve.

Air Valve Sending CQ

Once the air valve was connected to the cylinder, . . . things got MUCH more interesting, and VIOLENT!

For this test, the air pressure was set at about 5 psi (34.5 kpa), for actual or normal operation the pressure will be about 40 psi (275 kpa). (I think I have the unit conversions correct)

Believe me, the following video does NOT fully capture the loudness or intensity of being - "ON THE AIR with AIR".

Pounding Out - CQ

Sorry for the rotated video (I will avoid that mistake with my future videos).

The test of the FET Driver? - It works great, . . . and I now know what it really means to "pound out" CQ. :-)

Tuesday, February 18, 2014

I took my Homebrew Mixer and Amplifier (see previous posts) to Jacks Homebrew Meeting tonight. I was hoping that Jack and his Lab could help provide meaningful measured performance data.

Jack has several very nice pieces of LAB Grade test equipment. Unfortunately, his tracking Oscillator-Spectrum Analyzer was missing a cable or not working, and could not be used.

But with other equipment, we were able to spot check performance with an HP Oscillator with Attenuator and a HP Frequency Sensitive Voltmeter with its Attenuator. The two attenuators agreed within 0.25db. We also had a standalone attenuator for gross signal level adjustment.

The results; the Amplifier measure about +30db gain from 1 to about 27MHz (which was the highest Freq the Oscillator was calibrated for). The Amplifier would "quiet" the Voltmeter with as little as 115dbm input signal. Actual Noise Figure performance was not measured. This overall measured results are similar to what LTSpice suggested, but less than I measured at a Load on my Oscilloscope. NOTE: I may be mixing in my mind; Power Gain with Voltage Gain performance data, I may need to rethink this measurement.

The Mixer performance data and method was less precise, and accurate measurement will have to wait until Jack has his other equipment working.

Friday, February 7, 2014

If you look at the schematic of the Mixer shown as used with LTSpice (see previous post), you will see the center of the lower transformer is not grounded. The original Farhan circuit grounds this point. When I first copied it, that is, created the circuit within LTSpice, I inadvertently forgot the ground connection.

But the LTSpice simulation worked as expected so I assumed the circuit was correct. Later while cross-checking the circuit for another issue, I noticed the forgotten ground. But, then when the transformer ground was connected, the LTSpice simulation did not show the expected 20MHz output via the FFT. Now I am really confused.

To allow the created circuit on the PCB to be tested with and without the transformer grounded, I inserted two Zero Ohm resistors that will be left out of the ground path for initial testing. I am sure they will be needed as I am sure Farhan knows his circuit much better than I.

Without the DC ground path I wonder how the LTSpice solution is discharging the electron build up from the effects of rectification at the junction of the JFET. Perhaps the other JFET junction provides a DC leak to ground. Regardless, I have something to do experiments with.

Also, I laid out the circuit board for smaller coil forms (BN-43-2402), but I could not find my 36 AWG wire to wind the cores. When found, I plan to load another PCB with the small cores.

UPDATE: Feb 14, 2014
I found a role of #26 AWG wire and decided to try to wind a tri-filer winding on the very small BN-43-2402 binocular core. I was only able to get about three turns on the core, I needs 8 turns for this project. I will continue looking for my much smaller wire.

UPDATE
I had previously mis-posted the part number of the Binocular Cores that I have used. the number and links are now correct.

Thursday, February 6, 2014

With a little work, I was able to shrink the PCB for my mixer (see previous post). The smaller coils, smaller JFET foot prints allow for optimization and part placement. The PCB has be reduced to 1.0 x 1.4 inches.

Etched, Solder Wiped and Drilled,Ready for Cut and Parts

The Completed MixerThe Two J310 JFETs are located in the CenterJFET Bias Circuit is on the LeftThe Two Coils are, of course, Obvious

Testing will follow as tests are devised.

I spent some more time with LTSpice. I changed the LO from a small sign wave to a 70mV square wave, it works much better. Then I could correctly observed the results via an FFT plot. For the simulation I used a frequency of 27MHz for the LO and 7MHz for the input signal. As can be seen in the FFT plot, the output contains the desired 20MHz signal and lots of higher harmonics. The 20MHz peak is the difference (27 - 7 = 20) and the 34MHz peak is the sum (27 + 7 = 34), the 27MHz LO is suppressed, as it should be.

Lower Left is the Farhan Mixer CircuitUpper Left is the Full FFT PlotUpper Right is the Expanded FFT Plot Centered on 27MHzLower Right is the Output Plot at the 50 ohm Load

Note: I think a normal Diode Ring Mixer requires about 700mV P (~7dbm) of LO input, or something greater than the 0.6v to turn on the diodes. I think for this mixer the JFETs can use a much smaller LO signal.

Wednesday, February 5, 2014

I have been wanting a simple Mixer Circuit for experiments in my electronics shop, and most recently for use with my Small Signal Amplifier (see previous post).

Alan - K6ZY at the last Puget Sound QRP (pQRP) meeting mentioned he was considering building a Homebrew Farhan VU2ESE Minima Transceiver project. After looking at the schematic for the Mimina, it appears that the Mixer Circuit would work very nicely as an stand-alone mixer for my experimental use. Very good documentation of the mixer's operation is provided by Farhan. The mixer used in the Minima has been dubbed the “KISS Mixer” by Chris Trask in his paper.

Minima Mixer Circuit in LTSpice(See file below)

To play with the idea of building an experimental mixer, I created a LTSpice simulation circuit to start my understanding of it operation.

So far I have not gleamed much information from the LTSpice simulation, but maybe I have something wrong with my circuit; bias, configuration, signal levels or expected output. Or, maybe LTSpice can not deal with mixed signals correctly, but I will continue working with LTSpice to learn more.

In preparation for my experiment and use of the Mixer Circuit, I have started the initial design of a PCB using DipTrace. Keeping with my ever present Goal of building electronic projects as small as I can, this initial design is larger than I think the final design will be, as I have currently configured it to used larger components than necessary. I think I can find small SOT-23 J310 JFET packages and smaller transformer cores. But this is a start.

Mixer Layout in DipTrace

I have configured the PCB as a single-sided circuit as it can be produced with simple Homebrew Toner Transfer Method. But perhaps, using double-sided will be best for reducing the over all size.

This mixer as a stand-alone device will be very useful in my electronics shop - Thanks Farhan.

Monday, February 3, 2014

I connected a reasonably calibrated 100uVolt RMS 50 ohm RF source to the input of my Small Signal RF Amp (see previous post). The output of the amp with a 50 ohm load was 20mVolts P, for a Gain of about 43db. Similar gain was seen at frequencies from 1 to 26 MegHz, with more on the lower frequencies, and less gain on higher frequencies. Currently, I do not have proper equipment to do a complete characterization of the amp.

Test Setup Show 20mVolts RMSWith 50 ohm Load On the Left100uVolt RMS Source from Sig Gen on the Right

I am a happy camper !

I think this amp will be used in several of my future projects, stay tuned!

The amp was originally designed for 12 volt supply, but I think a local zener diode regulation at 9 volts (or battery) will be useful for use within an actual project. Regulation will help avoid supply side noise from effecting the performance. The above test was done with a 9 volt battery.

UPDATE: Feb 04, 2014
I checked with the LTSpice Simulation, it suggests with 100uV RMS at 10MHz as input, I should only see 5mVolts P at the output. I am measuring about 4 times that as reported above, I wonder why?

Sunday, February 2, 2014

I corrected the layout of my Small Signal RF Amp as described in the previous post. This time I printed both the Front and Back side of the layout for use with the Toner Transfer Method.

This photo of the etched board was taken with back light, which shows good alignment with the front side hole images. Actually, only two holes will be drilled, they will be used to mount the power header.

View through the board via Back-Light

The Front of the Completed Board

The Back

Now for some performance tests.

But, for a quick test, I tried it as an input amplifier for a receiver, it passes signals, but more proper testing is needed. I am sure it will work best as an IF Amplifier.

Some Project Background

For a transfer, I normally use a modified laminator (modified to set higher temperature at 350F or 176C). But for this project I wanted to use the Clothes Iron method. My previous attempts to use the clothes iron were less than satisfactory, probable because I did not know what I was doing. Now with more knowledge and understanding of what makes for a good transfer, the results speak for it self. It is all about; Board Preparation, the right Temperature, Time, Pressure and Transfer Media. I will go into the details in a future post.

UPDATE: Feb 2, 2014
I just noticed an "0" was missing from the date/time on the back side image, but then it was missing on the upper back-light image also, which became the finished board. It must have just not transferred - go figure?

Saturday, February 1, 2014

I have had an idea for an small signal RF Amplifier that I want to try. LTSpice suggest the gain will be good and the linearity appears to be good, or at least that is what the LTSpice FFT plot suggests. This Amp is similar to those that I have used in my Digital 15 Watt Power Amp, but biased for linearity.

My Goal for this circuit is Low Part Count and good Performance. And, as always, a major goal for all of my projects is to make them as Small as my Eyes, Nerves and Methods permits. Yes, I know there are published circuits that may work better, but this is my attempt.

This layout is a single-sided board, with ground plane on the backside. The SMA connectors, connects all ground planes together at the circuit board edge. A layout for a commercial manufactured board would include many via's to assist with ground plane connections.

The first Toner Transfer (TT) version of the circuit had a schematic error (my screw up, I was just to quick on the print button). The second version is shown here.

Etched andToner was Removed Underwater with Scotch Bright

Solder Wipedand Ready for Cut and Part Installation

But alas, I made the power pads too small and they pulled up with only a slight tug on the cable, layout will need to be re-done.